Like coal, oil and natural gas, uranium is an energy resource that must be processed through a series of steps to produce an efficient fuel for use in the generation of electricity. Each fuel has its own distinctive fuel cycle; however, the uranium or ‘nuclear fuel cycle’ is more complex than the others. To prepare uranium for use in a nuclear reactor, it undergoes the steps of mining and milling, conversion, enrichment and fuel fabrication. These steps make up the ‘front end’ of the nuclear fuel cycle. After uranium has been used in a reactor to produce electricity it is known as ‘spent fuel’ and may undergo a further series of steps including temporary storage, reprocessing, and recycling before eventual disposal as waste. Collectively these steps are known as the ‘back end’ of the fuel cycle.
Current processing methods require uranium to be in the form of a gas before it can be enriched, the natural uranium from the processed ore is converted into the gas uranium hexafluoride (UF6). Enriched UF6 is transported to a fuel fabrication plant where it is converted to enriched uranium dioxide (UO2) powder (typically 3-4% U-235 with the remaining uranium mostly U-238) and pressed into small pellets. These pellets are inserted into thin tubes, usually of a zirconium alloy (zircalloy) or stainless steel, to form fuel rods. The rods are then sealed and assembled in clusters to form fuel elements or assemblies for use in the core of the nuclear reactor. Some 25 tonnes of fresh fuel is required each year by a 1000 MWe reactor.
Spent fuel assemblies taken from the reactor core are highly radioactive and give off a lot of heat. They are therefore stored in special ponds, which are usually located at the reactor site, to allow both their heat and radioactivity to decrease. The water in the ponds serves the dual purpose of acting as a barrier against radiation and dispersing the heat from the spent fuel. Spent fuel can be stored safely in these ponds for long periods. It can also be dry stored in engineered facilities. However, both kinds of storage are intended only as an interim step before the spent fuel is either reprocessed or sent to final disposal. The longer it is stored, the easier it is to handle, due to decay of radioactivity. There are two alternatives for spent fuel: 1) reprocessing to recover the usable portion of it, and 2) long-term storage and final disposal without reprocessing.
The present inventors have determined that microstructured fuels contain fissile material structures with micrometer-scale dimensions dispersed in a matrix material. Most fission products escape from the fissile material structures and come to rest in the matrix material. This can allow a much cheaper separation of the fission products and fissile material, after the fuel is removed from the power system and allowed to cool for a number of years.